The present invention relates to a cemented carbide insert and method of manufacture via sintering, wherein the as-formed sintered insert is free of binder phase surface layer. The surface of the insert thus has a binder content similar to or less than the binder content of the bulk phase.
During the almost 70 years that cemented carbides have been used for metal cutting, constant improvements have been made in the field of cemented carbide insert production. The increasing use of powder compaction to near net shape has led to a need for cemented carbide grades with well defined surfaces that are suited for physical vapour deposition (PVD), chemical vapour deposition (CVD) and medium temperature chemical vapour deposition (MTCVD) coating without pre-treatment. Such inserts are commonly made of a metallic carbide, normally WC, generally with the addition of carbides of other metals such as Nb, Ti, Ta, etc. and a metallic binder phase of cobalt. To increase wear resistance, it is common to apply a thin layer of one or more wear resistant materials such as TiC, TiN, Al2O3 etc. to the surface.
A problem common to many cemented carbide grades is the presence of a binder phase surface layer partly or fully covering the outer tungsten carbide grains. This unwanted binder phase layer, which can be greater than 1 μm thick, develops during the sintering step. If a binder phase layer is present on the surface, it can have a negative effect on CVD and PVD processes, resulting in layers with inferior mechanical properties and insufficient adherence of the coating to the substrate. The binder phase layer must therefore be removed before carrying out the deposition process. The occurrence of the binder layer correlates with tungsten carbide grain size. In general, as grain size decreases below about 2 μm, and particularly below about 1.5 μm, binder phase becomes more prevalent on the surface and hence more problematic with respect to mechanical properties and coating adhesion. Fine and submicron grades of cemented carbide are particularly subject to surface binder formation.
While the art has addressed the problem of binder phase formation in a variety of ways, most of these can be grouped into two broad categories. In a first category are those methods that prevent the binder phase from initially forming. In a second category are methods that do allow the binder phase to form initially on the surface, and then attempt to remove the binder by mechanical or chemical means.
As previously stated, a binder phase surface layer tends to occur in cemented carbide grades with grain sizes smaller than about 2 μm. Hence simply by keeping grain size above the limit for binder phase formation, the entire problem is avoided. Larger grain sizes, however, carry their own disadvantages. For example, at a given binder level in the bulk cemented carbide, the room temperature (RT) hardness, i.e., resistance to plastic deformation, decreases with increasing grain size. In like manner, to obtain a given RT hardness level, the level of binder must be decreased as the tungsten carbide grain size is increased. Since toughness increases with higher levels of binder, the net effect is that either RT hardness or toughness usually suffers as grain size increases.
This trade-off between hardness and toughness at larger grain size is addressed in a unique way by U.S. Pat. No. 6,333,100 which teaches the addition of high levels of cubic carbide (4-12 wt. %) to a powder composition. The resulting sintered carbide insert of this patent has a cobalt binder phase enriched and essentially cubic carbide free surface zone of a certain thickness and composition along either side of a cutting edge. This, combined with the optimisation of cubic carbide near the cutting edge, contributes to simultaneous improvements in resistance to plastic deformation and toughness. At the same time, because the actual surface of the insert (as opposed to a surface zone which is immediately below the surface) is free of excess binder phase because of inter alia the large gain size used, coatings will remain adhered to the insert and mechanical properties will be maintained.
While the teachings of this patent clearly advance the art by improving the trade-off of using large tungsten carbide grains in cemented carbides, it does so at the expense of directly addressing the problem of binder phase formation with smaller grain sizes. As a result, the advantages of using smaller grain sizes are foregone. Moreover, the patent requires a number of specific features to be combined which may require careful monitoring, such as the specified certain thickness of the binder phase enriched surface zone. This may in some instances increase production costs compared to processes with less stringent composition and geometry requirements.
Binder phase formation can also be suppressed by tightly controlling cooling temperature, as described in U.S. Pat. No. 6,207,102, which teaches rapid cooling of the cemented carbide after sintering. The rapid cooling produces a surface with no binder phase layer. This method, while effective, requires specialised equipment and monitoring of the cooling step to produce the desired result.
Methods of the second category, that is, those methods, which allow a binder layer to initially form, and then attempt to remove it, include steps such as mechanical removal by blasting. Blasting, however, is difficult to control because of the inability to accurately control blasting depth. This in turn leads to increased scatter in the properties of the coated insert end product and damage to the hard constituent grain of the substrate surface. However, in U.S. Pat. No. 6,132,293, it is disclosed that blasting with fine particles gives an even removal of the binder phase layer without damaging the hard constituent grains.
Alternatively, chemical or electrolytic methods could be used to remove the binder layer. However these methods remove more than just the cobalt surface layer. They also result in deep penetration, particularly in areas close to the insert edge. This may create an undesired porosity between layer and substrate at the same time the binder layer may partly remain in other areas of the insert.
A further drawback of the above mentioned prior art methods is that they require additional production steps to remove the surface binder layer and for that reason are less attractive for large scale production. It would be desirable if sintering could be performed in such a way that no binder phase layer is formed.